The general goal of this proposal is to employ fluorometric techniques to monitor membrane potentials in Ehrlich ascites tumor cells. Preliminary studies indicate that the fluorescent intensity of the dye 3,3'-dipropyl-2,2'-thiadicarbocyanine iodide is sensitive to membrane potential in these cells. The demonstration of the efficacy of the fluorometric methods with ascites cells would extend the range of cells on which the method may be employed to include cells of typical eucaryotic structure. Recently estimations of the magnitude of this potential under different conditions have become critical in the testing of the Na ion-gradient hypothesis (as modified to include electrical as well as the chemical gradient) of amino acid transport. Electrophysiological studies on ascited cells have given a wide range of possible values and have been questionable because of the relatively small size of these cells. Furthermore, it has not been possible to use electrophysiological techniques to follow the potential in a cell for any length of time. The specific aims of the proposal are: 1) to continue to test the postulate that changes in fluorescence parallel and hence monitor membrane potentials in Ehrlich ascites tumor cells. 2) to determine the contribution of the passive movements of various ions and the activity of ion pumps of the membrane potential. 3) to calibrate the system so that the membrane potential of the cells in a variety of conditions may be determined. 4) to determine whether or not the initial rate of Na ion-dependent amino acid transport is the same for a constant Na ion electrochemical gradient composed of various combinations of Na ion chemical and electrical gradients. 5) to determine whether or not the energy available in the Na ion-gradient is sufficient to account for the active transport of amino acids under a variety of conditions.